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Highly porous hematite nanorods prepared via direct spray precipitation method
Materials Letters 117 (2014) 279–282
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Highly porous hematite nanorods prepared via direct spray precipitation method Dean Cardillo a,b, Moeava Tehei b,c, Michael Lerch b,d, Stéphanie Corde d,e, Anatoly Rosenfeld d, Konstantin Konstantinov a,n a
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia Illawarra Health and Medical Research Institute, University of Wollongong, NSW 2500, Australia c School of Chemisty, Faculty of Science, Medicine and Health, University of Wollongong, Wollongong, NSW 2500, Australia d Centre for Medical Radiation Physics, University of Wollongong, NSW 2500, Australia e Radiation Oncology Department, Prince of Wales Hospital, Randwick, NSW 2031, Australia b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 August 2013 Accepted 16 November 2013 Available online 8 December 2013
Hematite nanoparticles can be formed through a variety of synthesis routes, each of which can substantially affect the properties of the resulting product. In this article we report the use of a facile synthesis route, direct spray precipitation, and compare the performance of these α-Fe2O3 products with those obtained through standard precipitation method. Through this new method, we obtain a nanocrystalline α-FeO(OH) precursor material, while an amorphous material is obtained from precipitation route. The highly efficient, low temperature (300 1C) calcination of this nanocrystalline precursor results in the formation of hematite nanorods with a diameter between 20 and 30 nm. These nanorods exhibit an extremely high surface area (up to 166 m2 g 1) attributed to their morphology and apparent nanoporosity. This very high surface area coupled with higher attenuation in the ultraviolet-visible with a low temperature synthesis give this material potential for further investigation as photocatalyst or energy storage material. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Ceramics Porous materials Optical materials and properties Nanocrystalline materials
1. Introduction Nanoparticles of hematite (α-Fe2O3) are an example of a nanomaterial that can be tailored to exhibit favourable magnetic [1,2], optical [3–5] electrochemical [6–8] and photocatalytic [9,10] properties for a variety of applications [11]. Hematite nanoparticles have been reported to be produced from many different routes available such as hydrothermal [2,7,12,13], high-energy ball milling [14], spray pyrolysis [15], precipitation [3,5], thermal decomposition [4] along with more advanced techniques [1,8,9,16,17]. However, some of these methods required a long time to achieve the desired size and properties (hydrothermal, ball-milling) or very high temperatures (spray pyrolysis). In this article we introduce an efficient method of synthesising very high specific surface area α-Fe2O3 nanoparticles namely, direct spray precipitation, which involves the rapid delivery of a Fe3 þ solution into a solution of strong base via two-fluid spray nozzle. This has the advantages of combining the high-yield and possibility for thermally efficient (low temperature, 300 1C) nature of traditional α-Fe2O3 precipitation with the smaller droplet size and kinetic n
Corresponding author. Tel.: þ 61 2 4221 5765; fax: þ61 2 4221 5731. E-mail address: [email protected] (K. Konstantinov).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.131
effects associated with the increased velocity of these droplets, such as is featured in spray pyrolysis. This results in the formation of nanorods of crystalline α-FeO(OH) in a significantly more efficient process that previously reported [18], which after undergoing low temperature decomposition results in α-Fe2O3 nanorods that initially exhibit extremely high surface area and ultravioletvisible optical absorbance as compared to traditionally precipitated α-Fe2O3. However this high porosity is lost upon heating and unlike traditionally precipitated material this further heating does not result in a significant blue shift of absorption spectra. 2. Experimental 2.1. Synthesis Iron(III) nitrate hexahydrate ðFeðNO3 Þ3 9H2 O, Sigma–Aldrich 99%) and sodium hydroxide (NaOH, Sigma–Aldrich Z 97%) were dissolved into deionized water to create 0.15 M Fe(NO3)3 and 1.5 M NaOH solutions. Under the influence of magnetic stirrer bar, NaOH solution was added to the Fe(NO3)3 solution drop-wise via Pasteur pipette. In the direct spray precipitation method, solutions of the same concentration were used, except a spray nozzle and peristaltic pump were used to spray the Fe3 þ solution into the basic
280
D. Cardillo et al. / Materials Letters 117 (2014) 279–282
bandgaps were determined from constructing Tauc plots [19], 1 plotting (αhν)n ¼K(hν Eg ), where n ¼ for indirect and n ¼2 for 2 direct transitions, while α [3]:
α¼
2:303 103 Aρ cℓ
ð1Þ
and ρ, the density of hematite is set to 5.25 g cm 3, c is the concentration of the suspension in g mL 1 and ℓ is the path length in cm. Band gap Eg values quoted in this study refer to those of direct band gap n ¼2 transitions, as this featured a better linear fit.
3. Results and discussion
Fig. 1. Diagram of reaction set-up for direct spray precipitation method.
NaOH solution (Fig. 1). Both methods resulted in a red-brown precipitate which were allowed to rest overnight, resulting in the partial separation of the solid and liquid phases. The spray precipitation precursor underwent ageing, changing in colour from brown-red to yellow. After decanting the upper liquid layer, the precipitants were separated, washed and resuspended multiple times at 11,000 RPM for 5 min via centrifugation. The resulting solid precursor materials were then dried in an oven for 3 h at 90 1C. This precursor material was then annealed at 300 1C, 400 1C and 500 1C in a tube furnace (Thermotech, Haugesund, Norway) for 4 h. 2.2. X-ray diffraction Powder diffraction data was obtained for all samples on a Enhanced Mini-Materials Analyser (EMMA) X-ray Diffractometer (XRD) (GBC Scientific Equipment, Melbourne, Australia) between 20 and 90 1 at 2:0001 min 1 and a step size of 0.0201. Mean crystalline size was calculated using Scherrer's Formula, while lattice parameters were calculated using MAUD software. 2.3. Brunauer–Emmett–Teller analysis The specific surface area of all samples was investigated using a NOVA 1000 (Quantachrome, Boynton Beach, Florida, USA) high speed gas sorption analyser. N2 gas was used as the adsorbate at the temperature of liquid nitrogen on samples previously degassed at 130 1C overnight. 2.4. Scanning transmission electron microscopy TED (transmitted electron detector) images were then taken using a JSM-7500FA cold Field Emission Gun Scanning Electron Microscope (FEGSEM) (JEOL, Tokyo, Japan). The operating parameters include an accelerating voltage between 17 and 25 kV at a working distance of 8 mm, a beam current of 10 μA and a spot size setting of 8. Carbon film coated copper grids were dipped into suspensions of each sample in ethanol. 2.5. Ultraviolet-visible absorption spectroscopy Suspensions of 0.05 mg.mL 1 of were sonicated for 2–3 h after which 250 and 1000 nm was obtained using ometer (Shimadzu, Kyoto, Japan).
nanoparticles in ethanol the absorbance between a UV-1800 SpectrophotDirect-transition optical
The diffraction patterns obtained for traditionally precipitated samples (Fig. 2(a)) show that while an amorphous phase with no strong peaks was obtained as the precursor material (black pattern), α-Fe2O3 was confirmed as the primary phase of all other samples. The yellow precursor obtained though the direct spray method exhibited nanoscale crystallinity with a number of very broad peaks corresponding to a primary goethite phase (α-FeO (OH)), PDF card no. 00-003-0249) with lattice parameters a ¼ 4:6139 7 0:0009 Å, b ¼ 9:941 7 0:001 Å and c ¼ 3:0224 7 0:0004 Å. Hematite nanoparticles produced through this direct spray precipitation initially exhibit a larger degree of peak broadening (Fig. 2 and Table 1) and as such values obtained for mean crystalline size for this method give smaller values than those calculated for standard precipitated samples (calculated using (1 0 4) peak) for the same calcination temperature. Both preparation methods also show a decrease in lattice parameters a with increasing calcination temperature above 500 1C (Fig. 2(d)), while c shows a steady decrease with increasing temperature for the samples prepared though standard precipitation. Lattice parameter c stays roughly constant for samples obtained through spray precipitation. Table 1 shows that at low sintering temperature, the use of direct spray precipitation results in particles with a much higher specific surface area, which is 166 m2 g 1 for a mean crystalline size of 10 nm, compared to 55 m2 g 1 and 21 nm size for traditionally precipitated hematite nanoparticles. This is due to this temperature being sufficiently high enough to drive off entrapped water within the goethite structure [20]. As the annealing temperature increased, this property is quickly lost as thermally driven crystal growth engulfs the apparent nanoporosity, which can be seen through the inverse proportionality in Fig. 2 (c). This same porosity is not evident in the α-Fe2O3 nanoparticles formed through traditional precipitation. Scanning transmission images show that the mean crystalline size of nanoparticles formed through the precipitation method (Fig. 3(a) and (b)) is in good agreement with values calculated from the X-ray diffraction patterns. They also depict a reasonably uniform particle size distribution, with the morphology of these particles being spherical. However, precursor and calcinated samples formed through direct spray precipitation method (Fig. 3(c)–(f)) have the morphology of nanorods, a particle geometry which has been previously obtained through other methods of synthesis [21–23]. These nanorods have a diameter which agrees with the calculated mean particles sizes obtained through the use of Scherrer's formula on the (1 0 4) peak, while they have a length that is greater than 100 nm. The large values obtained for specific surface area obtained from BET analysis for the 300 1C sample (Fig. 3(d)) can be explained from this morphology and the evident nanoporosity can be seen in the STEM images (Fig. 3(d)), which can be seen to become less apparent with an increase in firing temperature. It is a
D. Cardillo et al. / Materials Letters 117 (2014) 279–282
281
500oC o
500 C o 400 C o 400 C
300oC
300oC
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Angle 2θ (o)
Angle 2θ (o)
180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
50
d
40 30 20 10 0 300
400
500 o
AnnealingTemperature( C)
13.775 5.036 13.750 5.034
13.725
5.032 a 5.030 c 5.028
13.700 Precipitation Spray Precipitation
13.675
Precipitation Spray Precipitation
13.650
5.026
13.625 300
400
Annealing Temperature (oC)
500
Fig. 2. X-ray diffraction patterns for (a) α-Fe2O3 nanoparticles formed through precipitation and (b) α-Fe2O3 nanorods formed through direct spray method, with the bottom patterns corresponding to precursor materials. Relationship between (c) mean crystalline size (squares) and specific surface area (triangles) for spray precipitation (white) and traditionally precipitated (black) samples, and (d) lattice parameters with annealing temperature are also shown.
Fig. 3. Scanning Transmission Electron Microscope images of α-Fe2O3 nanoparticles formed through traditional precipitation at (a) 400 1C and (b) 500 1C, and (c) precursor formed through spray precipitation and nanorods after calcination at (d) 300 1C, (e) 400 1C, and (f) 500 1C.
possibility that the velocity of the droplets, as a result of coming from the spray nozzle, had an influence on causing preferential growth of these nanoparticles in the (1 1 0) plane, while aiding in
formation of highly crystalline precursor. This morphology can also be seen in the different relative intensity and peak broadening between the (1 0 4) and (1 1 0) peaks in X-ray diffraction patterns
282
D. Cardillo et al. / Materials Letters 117 (2014) 279–282 Precipitation 300oC 400oC 500oC
2.50E+008
α (cm-1)
2.00E+008
Direct Spray Precipitation 300oC 400oC 500oC
1.50E+008
while the evident large growth in the (1 1 0) crystal plane due to rod morphology dominates the effects on the spectra of spray precipitated material, with decreasing value for optical bandgap obtained with increasing calcination temperature (Table 1) due to further crystal growth.
4. Conclusions
1.00E+008
5.00E+007
0.00E+000 300
400
500
600
700
800
900
1000
Wavelength (nm) Fig. 4. Ultraviolet-visible absorption spectra for α-Fe2O3 nanoparticles formed through precipitation and spray-assisted precipitation routes in ethanol.
Table 1 Values of mean crystalline size obtained by application of the Scherrer formula, surface area values found through BET analysis, along with optical bandgap found through UV-vis absorption spectrophotometry. Method
Annealing temperature (1C)
Mean crystalline size (nm)
Surface area (m2 g 1)
Precipitation
300 400 500
21 28 38
54.91 33.22 18.09
2.737 0.03 2.60 7 0.04 3.417 0.02
Spray precipitation 300 400 500
10 22 31
165.98 30.80 16.67
2.727 0.02 2.62 7 0.02 2.55 7 0.02
Bandgap (eV)
of traditionally precipitated (Fig. 2(a)) and direct spray (Fig. 2(b)) samples. This helps to explain the lack of peak broadening of the representative peak in the X-ray diffraction pattern of these materials compared to the same peak in the diffraction pattern of the samples formed through standard precipitation (Fig. 2(a)). All prepared samples show (Fig. 4 and Table 1) some degree of blue-shift in their optical absorption spectra relative to the bulk value of 2.1–2.2 eV for hematite, which is expected as a result of quantum confinement that comes with a nanoscale grain size [24]. The absorption bands within the spectra of hematite have been shown to have strength relative to the degree of order present within its crystal structure [3], as these strong absorption bands are due to Fe3 þ -O 2 charge transfer. As such, it seems fairly evident that the nanorods formed through spray precipitation initially exhibit higher attenuation as they have come from a crystalline α-FeO(OH) precursor, while those formed through a precipitation method do not and in calcination, more thermal energy is needed for the re-arrangement of the hematite lattice to get highly crystalline nanomaterials. However, as 500 1C is reached for traditional precipitation the order present reaches critical point resulting in a large blue-shift (3.417 0.02 eV) in absorption edge,
The novel synthesis route that we report in this study, direct spray-assisted precipitation, can be used for effective and thermally efficient low temperature method of fabricating hematite nanorods with extremely high surface area and optical attenuation as compared to traditionally precipitated hematite nanoparticles. These interesting properties are facilitated through the formation of a nanorods of α-FeO(OH) nanocrystalline precursor as a result of using this spray precipitation method. This differing morphology is obtained as a result of smaller size and higher energy of the droplets as emitted from the two-fluid spray nozzle, as compared to those added to the reaction mixture via pipette. This method of synthesis has the potential to augment the properties of other ceramic nanomaterials that can be formed through similar precipitation routes. Such high porosity, rod morphology, and high attenuation for the low temperature spray-precipitated sample make this material attractive for future investigation as materials for application in water-splitting and photocatalysis. References [1] Zhang X, Li Q. Mater Lett 2008;62(67):988–90. [2] Sun Q, Gang Lu X, Ying Liang G. Mater Lett 2010;64(18):2006–8. [3] Truffault L, Choquenet B, Konstantinov K, Devers T, Couteau C, Coiffard LJM. J Nanosci Nanotechnol 2010;10:1–8. [4] Al-Gaashani R, Radiman S, Tabet N, Daud A. J Alloy Compd 2013;550. [5] Cardillo D, Konstantinov K, Devers T. Mater Res Bull 2013;48(11):4521–5. [6] Zhu M, Wang Y, Meng D, Qin X, Diao G. J Phys Chem C 2012;116. [7] Liu Z, Lv B, Wu D, Sun Y, Xu Y. Particuology 2013;11(3):327–33. [8] Ang WA, Gupta N, Prasanth R, Hng HH, Madhavi S. J Mater Res 2013;28 (6):824–31. [9] Liang H, Wang Z. Mater Lett 2013;96(0):12–5. [10] Ahmmad B, Leonard K, Islam MS, Kurawaki J, Muruganandham M, Ohkubo Takahiro, et al. Adv Powder Technol 2013;24(1):160–7. [11] Tartaj P, Morales MP, Gonzalez-Carreño T, Veintemillas-Verdaguer S, Serna CJ. Adv Mater 2011;23. [12] Xu Y, Yang S, Zhang G, Sun Y, Gao D, Sun Y. Mater Lett 2011;65(12):1911–4. [13] Zhu W, Cui X, Wang L, Liu T, Zhang Q. Mater Lett 2011;65(6):1003–6. [14] Arbain R, Othman M, Palaniandy S. Miner Eng 2011;24(1):1–9. [15] Strobel R, Pratsinis S. Phys Chem Chem Phys 2011;13(20):9246–52. [16] Bo Cao R, Quan Chen X, Hao Shen W, Long Z. Mater Lett 2011;65(21–22): 3298–3300. [17] Liu J, Liang C, Zhang H, Tian Z, Zhang S. J Phys Chem 2012;116(8):4986–92. [18] Zhang H, Bayne M, Fernando S, Legg B, Zhu M, Penn RL, Banfield JF. J Phys Chem C 2011;115(36):17704–10. [19] Tauc J. Amorphous and Liquid Semiconductors, London: Plenum Press; 1974. [20] Naono H, Nakai K, Sueyoshi T, Yagi H. J Colloid Interface Sci 1987;120 (2):439–50. [21] Lian S, Wang E, Kang Z, Bai Y, Gao L, Jiang M, Hu C, Xu L. Solid State Commun 2004;129(8):485–90. [22] Wu C, Yin P, Zhu X, OuYang C, Xie Y. J Phys Chem B 2006;110(36):17806–12 PMID: 16956266. [23] Guo P, Wei Z, Wang B, Ding Y, Li H, Zhang G, Zhao X. Colloid Surf A 2011;380 (13):234–40. [24] Zeng S, Tang K, Li T. J Colloid Interf Sci 2007;312(2):513–21.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Highly porous hematite nanorods prepared via direct spray precipitation method Dean Cardillo a,b, Moeava Tehei b,c, Michael Lerch b,d, Stéphanie Corde d,e, Anatoly Rosenfeld d, Konstantin Konstantinov a,n a
Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2500, Australia Illawarra Health and Medical Research Institute, University of Wollongong, NSW 2500, Australia c School of Chemisty, Faculty of Science, Medicine and Health, University of Wollongong, Wollongong, NSW 2500, Australia d Centre for Medical Radiation Physics, University of Wollongong, NSW 2500, Australia e Radiation Oncology Department, Prince of Wales Hospital, Randwick, NSW 2031, Australia b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 August 2013 Accepted 16 November 2013 Available online 8 December 2013
Hematite nanoparticles can be formed through a variety of synthesis routes, each of which can substantially affect the properties of the resulting product. In this article we report the use of a facile synthesis route, direct spray precipitation, and compare the performance of these α-Fe2O3 products with those obtained through standard precipitation method. Through this new method, we obtain a nanocrystalline α-FeO(OH) precursor material, while an amorphous material is obtained from precipitation route. The highly efficient, low temperature (300 1C) calcination of this nanocrystalline precursor results in the formation of hematite nanorods with a diameter between 20 and 30 nm. These nanorods exhibit an extremely high surface area (up to 166 m2 g 1) attributed to their morphology and apparent nanoporosity. This very high surface area coupled with higher attenuation in the ultraviolet-visible with a low temperature synthesis give this material potential for further investigation as photocatalyst or energy storage material. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Ceramics Porous materials Optical materials and properties Nanocrystalline materials
1. Introduction Nanoparticles of hematite (α-Fe2O3) are an example of a nanomaterial that can be tailored to exhibit favourable magnetic [1,2], optical [3–5] electrochemical [6–8] and photocatalytic [9,10] properties for a variety of applications [11]. Hematite nanoparticles have been reported to be produced from many different routes available such as hydrothermal [2,7,12,13], high-energy ball milling [14], spray pyrolysis [15], precipitation [3,5], thermal decomposition [4] along with more advanced techniques [1,8,9,16,17]. However, some of these methods required a long time to achieve the desired size and properties (hydrothermal, ball-milling) or very high temperatures (spray pyrolysis). In this article we introduce an efficient method of synthesising very high specific surface area α-Fe2O3 nanoparticles namely, direct spray precipitation, which involves the rapid delivery of a Fe3 þ solution into a solution of strong base via two-fluid spray nozzle. This has the advantages of combining the high-yield and possibility for thermally efficient (low temperature, 300 1C) nature of traditional α-Fe2O3 precipitation with the smaller droplet size and kinetic n
Corresponding author. Tel.: þ 61 2 4221 5765; fax: þ61 2 4221 5731. E-mail address: [email protected] (K. Konstantinov).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.11.131
effects associated with the increased velocity of these droplets, such as is featured in spray pyrolysis. This results in the formation of nanorods of crystalline α-FeO(OH) in a significantly more efficient process that previously reported [18], which after undergoing low temperature decomposition results in α-Fe2O3 nanorods that initially exhibit extremely high surface area and ultravioletvisible optical absorbance as compared to traditionally precipitated α-Fe2O3. However this high porosity is lost upon heating and unlike traditionally precipitated material this further heating does not result in a significant blue shift of absorption spectra. 2. Experimental 2.1. Synthesis Iron(III) nitrate hexahydrate ðFeðNO3 Þ3 9H2 O, Sigma–Aldrich 99%) and sodium hydroxide (NaOH, Sigma–Aldrich Z 97%) were dissolved into deionized water to create 0.15 M Fe(NO3)3 and 1.5 M NaOH solutions. Under the influence of magnetic stirrer bar, NaOH solution was added to the Fe(NO3)3 solution drop-wise via Pasteur pipette. In the direct spray precipitation method, solutions of the same concentration were used, except a spray nozzle and peristaltic pump were used to spray the Fe3 þ solution into the basic
280
D. Cardillo et al. / Materials Letters 117 (2014) 279–282
bandgaps were determined from constructing Tauc plots [19], 1 plotting (αhν)n ¼K(hν Eg ), where n ¼ for indirect and n ¼2 for 2 direct transitions, while α [3]:
α¼
2:303 103 Aρ cℓ
ð1Þ
and ρ, the density of hematite is set to 5.25 g cm 3, c is the concentration of the suspension in g mL 1 and ℓ is the path length in cm. Band gap Eg values quoted in this study refer to those of direct band gap n ¼2 transitions, as this featured a better linear fit.
3. Results and discussion
Fig. 1. Diagram of reaction set-up for direct spray precipitation method.
NaOH solution (Fig. 1). Both methods resulted in a red-brown precipitate which were allowed to rest overnight, resulting in the partial separation of the solid and liquid phases. The spray precipitation precursor underwent ageing, changing in colour from brown-red to yellow. After decanting the upper liquid layer, the precipitants were separated, washed and resuspended multiple times at 11,000 RPM for 5 min via centrifugation. The resulting solid precursor materials were then dried in an oven for 3 h at 90 1C. This precursor material was then annealed at 300 1C, 400 1C and 500 1C in a tube furnace (Thermotech, Haugesund, Norway) for 4 h. 2.2. X-ray diffraction Powder diffraction data was obtained for all samples on a Enhanced Mini-Materials Analyser (EMMA) X-ray Diffractometer (XRD) (GBC Scientific Equipment, Melbourne, Australia) between 20 and 90 1 at 2:0001 min 1 and a step size of 0.0201. Mean crystalline size was calculated using Scherrer's Formula, while lattice parameters were calculated using MAUD software. 2.3. Brunauer–Emmett–Teller analysis The specific surface area of all samples was investigated using a NOVA 1000 (Quantachrome, Boynton Beach, Florida, USA) high speed gas sorption analyser. N2 gas was used as the adsorbate at the temperature of liquid nitrogen on samples previously degassed at 130 1C overnight. 2.4. Scanning transmission electron microscopy TED (transmitted electron detector) images were then taken using a JSM-7500FA cold Field Emission Gun Scanning Electron Microscope (FEGSEM) (JEOL, Tokyo, Japan). The operating parameters include an accelerating voltage between 17 and 25 kV at a working distance of 8 mm, a beam current of 10 μA and a spot size setting of 8. Carbon film coated copper grids were dipped into suspensions of each sample in ethanol. 2.5. Ultraviolet-visible absorption spectroscopy Suspensions of 0.05 mg.mL 1 of were sonicated for 2–3 h after which 250 and 1000 nm was obtained using ometer (Shimadzu, Kyoto, Japan).
nanoparticles in ethanol the absorbance between a UV-1800 SpectrophotDirect-transition optical
The diffraction patterns obtained for traditionally precipitated samples (Fig. 2(a)) show that while an amorphous phase with no strong peaks was obtained as the precursor material (black pattern), α-Fe2O3 was confirmed as the primary phase of all other samples. The yellow precursor obtained though the direct spray method exhibited nanoscale crystallinity with a number of very broad peaks corresponding to a primary goethite phase (α-FeO (OH)), PDF card no. 00-003-0249) with lattice parameters a ¼ 4:6139 7 0:0009 Å, b ¼ 9:941 7 0:001 Å and c ¼ 3:0224 7 0:0004 Å. Hematite nanoparticles produced through this direct spray precipitation initially exhibit a larger degree of peak broadening (Fig. 2 and Table 1) and as such values obtained for mean crystalline size for this method give smaller values than those calculated for standard precipitated samples (calculated using (1 0 4) peak) for the same calcination temperature. Both preparation methods also show a decrease in lattice parameters a with increasing calcination temperature above 500 1C (Fig. 2(d)), while c shows a steady decrease with increasing temperature for the samples prepared though standard precipitation. Lattice parameter c stays roughly constant for samples obtained through spray precipitation. Table 1 shows that at low sintering temperature, the use of direct spray precipitation results in particles with a much higher specific surface area, which is 166 m2 g 1 for a mean crystalline size of 10 nm, compared to 55 m2 g 1 and 21 nm size for traditionally precipitated hematite nanoparticles. This is due to this temperature being sufficiently high enough to drive off entrapped water within the goethite structure [20]. As the annealing temperature increased, this property is quickly lost as thermally driven crystal growth engulfs the apparent nanoporosity, which can be seen through the inverse proportionality in Fig. 2 (c). This same porosity is not evident in the α-Fe2O3 nanoparticles formed through traditional precipitation. Scanning transmission images show that the mean crystalline size of nanoparticles formed through the precipitation method (Fig. 3(a) and (b)) is in good agreement with values calculated from the X-ray diffraction patterns. They also depict a reasonably uniform particle size distribution, with the morphology of these particles being spherical. However, precursor and calcinated samples formed through direct spray precipitation method (Fig. 3(c)–(f)) have the morphology of nanorods, a particle geometry which has been previously obtained through other methods of synthesis [21–23]. These nanorods have a diameter which agrees with the calculated mean particles sizes obtained through the use of Scherrer's formula on the (1 0 4) peak, while they have a length that is greater than 100 nm. The large values obtained for specific surface area obtained from BET analysis for the 300 1C sample (Fig. 3(d)) can be explained from this morphology and the evident nanoporosity can be seen in the STEM images (Fig. 3(d)), which can be seen to become less apparent with an increase in firing temperature. It is a
D. Cardillo et al. / Materials Letters 117 (2014) 279–282
281
500oC o
500 C o 400 C o 400 C
300oC
300oC
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Angle 2θ (o)
Angle 2θ (o)
180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
50
d
40 30 20 10 0 300
400
500 o
AnnealingTemperature( C)
13.775 5.036 13.750 5.034
13.725
5.032 a 5.030 c 5.028
13.700 Precipitation Spray Precipitation
13.675
Precipitation Spray Precipitation
13.650
5.026
13.625 300
400
Annealing Temperature (oC)
500
Fig. 2. X-ray diffraction patterns for (a) α-Fe2O3 nanoparticles formed through precipitation and (b) α-Fe2O3 nanorods formed through direct spray method, with the bottom patterns corresponding to precursor materials. Relationship between (c) mean crystalline size (squares) and specific surface area (triangles) for spray precipitation (white) and traditionally precipitated (black) samples, and (d) lattice parameters with annealing temperature are also shown.
Fig. 3. Scanning Transmission Electron Microscope images of α-Fe2O3 nanoparticles formed through traditional precipitation at (a) 400 1C and (b) 500 1C, and (c) precursor formed through spray precipitation and nanorods after calcination at (d) 300 1C, (e) 400 1C, and (f) 500 1C.
possibility that the velocity of the droplets, as a result of coming from the spray nozzle, had an influence on causing preferential growth of these nanoparticles in the (1 1 0) plane, while aiding in
formation of highly crystalline precursor. This morphology can also be seen in the different relative intensity and peak broadening between the (1 0 4) and (1 1 0) peaks in X-ray diffraction patterns
282
D. Cardillo et al. / Materials Letters 117 (2014) 279–282 Precipitation 300oC 400oC 500oC
2.50E+008
α (cm-1)
2.00E+008
Direct Spray Precipitation 300oC 400oC 500oC
1.50E+008
while the evident large growth in the (1 1 0) crystal plane due to rod morphology dominates the effects on the spectra of spray precipitated material, with decreasing value for optical bandgap obtained with increasing calcination temperature (Table 1) due to further crystal growth.
4. Conclusions
1.00E+008
5.00E+007
0.00E+000 300
400
500
600
700
800
900
1000
Wavelength (nm) Fig. 4. Ultraviolet-visible absorption spectra for α-Fe2O3 nanoparticles formed through precipitation and spray-assisted precipitation routes in ethanol.
Table 1 Values of mean crystalline size obtained by application of the Scherrer formula, surface area values found through BET analysis, along with optical bandgap found through UV-vis absorption spectrophotometry. Method
Annealing temperature (1C)
Mean crystalline size (nm)
Surface area (m2 g 1)
Precipitation
300 400 500
21 28 38
54.91 33.22 18.09
2.737 0.03 2.60 7 0.04 3.417 0.02
Spray precipitation 300 400 500
10 22 31
165.98 30.80 16.67
2.727 0.02 2.62 7 0.02 2.55 7 0.02
Bandgap (eV)
of traditionally precipitated (Fig. 2(a)) and direct spray (Fig. 2(b)) samples. This helps to explain the lack of peak broadening of the representative peak in the X-ray diffraction pattern of these materials compared to the same peak in the diffraction pattern of the samples formed through standard precipitation (Fig. 2(a)). All prepared samples show (Fig. 4 and Table 1) some degree of blue-shift in their optical absorption spectra relative to the bulk value of 2.1–2.2 eV for hematite, which is expected as a result of quantum confinement that comes with a nanoscale grain size [24]. The absorption bands within the spectra of hematite have been shown to have strength relative to the degree of order present within its crystal structure [3], as these strong absorption bands are due to Fe3 þ -O 2 charge transfer. As such, it seems fairly evident that the nanorods formed through spray precipitation initially exhibit higher attenuation as they have come from a crystalline α-FeO(OH) precursor, while those formed through a precipitation method do not and in calcination, more thermal energy is needed for the re-arrangement of the hematite lattice to get highly crystalline nanomaterials. However, as 500 1C is reached for traditional precipitation the order present reaches critical point resulting in a large blue-shift (3.417 0.02 eV) in absorption edge,
The novel synthesis route that we report in this study, direct spray-assisted precipitation, can be used for effective and thermally efficient low temperature method of fabricating hematite nanorods with extremely high surface area and optical attenuation as compared to traditionally precipitated hematite nanoparticles. These interesting properties are facilitated through the formation of a nanorods of α-FeO(OH) nanocrystalline precursor as a result of using this spray precipitation method. This differing morphology is obtained as a result of smaller size and higher energy of the droplets as emitted from the two-fluid spray nozzle, as compared to those added to the reaction mixture via pipette. This method of synthesis has the potential to augment the properties of other ceramic nanomaterials that can be formed through similar precipitation routes. Such high porosity, rod morphology, and high attenuation for the low temperature spray-precipitated sample make this material attractive for future investigation as materials for application in water-splitting and photocatalysis. References [1] Zhang X, Li Q. Mater Lett 2008;62(67):988–90. [2] Sun Q, Gang Lu X, Ying Liang G. Mater Lett 2010;64(18):2006–8. [3] Truffault L, Choquenet B, Konstantinov K, Devers T, Couteau C, Coiffard LJM. J Nanosci Nanotechnol 2010;10:1–8. [4] Al-Gaashani R, Radiman S, Tabet N, Daud A. J Alloy Compd 2013;550. [5] Cardillo D, Konstantinov K, Devers T. Mater Res Bull 2013;48(11):4521–5. [6] Zhu M, Wang Y, Meng D, Qin X, Diao G. J Phys Chem C 2012;116. [7] Liu Z, Lv B, Wu D, Sun Y, Xu Y. Particuology 2013;11(3):327–33. [8] Ang WA, Gupta N, Prasanth R, Hng HH, Madhavi S. J Mater Res 2013;28 (6):824–31. [9] Liang H, Wang Z. Mater Lett 2013;96(0):12–5. [10] Ahmmad B, Leonard K, Islam MS, Kurawaki J, Muruganandham M, Ohkubo Takahiro, et al. Adv Powder Technol 2013;24(1):160–7. [11] Tartaj P, Morales MP, Gonzalez-Carreño T, Veintemillas-Verdaguer S, Serna CJ. Adv Mater 2011;23. [12] Xu Y, Yang S, Zhang G, Sun Y, Gao D, Sun Y. Mater Lett 2011;65(12):1911–4. [13] Zhu W, Cui X, Wang L, Liu T, Zhang Q. Mater Lett 2011;65(6):1003–6. [14] Arbain R, Othman M, Palaniandy S. Miner Eng 2011;24(1):1–9. [15] Strobel R, Pratsinis S. Phys Chem Chem Phys 2011;13(20):9246–52. [16] Bo Cao R, Quan Chen X, Hao Shen W, Long Z. Mater Lett 2011;65(21–22): 3298–3300. [17] Liu J, Liang C, Zhang H, Tian Z, Zhang S. J Phys Chem 2012;116(8):4986–92. [18] Zhang H, Bayne M, Fernando S, Legg B, Zhu M, Penn RL, Banfield JF. J Phys Chem C 2011;115(36):17704–10. [19] Tauc J. Amorphous and Liquid Semiconductors, London: Plenum Press; 1974. [20] Naono H, Nakai K, Sueyoshi T, Yagi H. J Colloid Interface Sci 1987;120 (2):439–50. [21] Lian S, Wang E, Kang Z, Bai Y, Gao L, Jiang M, Hu C, Xu L. Solid State Commun 2004;129(8):485–90. [22] Wu C, Yin P, Zhu X, OuYang C, Xie Y. J Phys Chem B 2006;110(36):17806–12 PMID: 16956266. [23] Guo P, Wei Z, Wang B, Ding Y, Li H, Zhang G, Zhao X. Colloid Surf A 2011;380 (13):234–40. [24] Zeng S, Tang K, Li T. J Colloid Interf Sci 2007;312(2):513–21.